1. Field of the Invention
This invention relates to a process for preparing urethanes by reacting a solution of a nitrogen-containing organic compound and a hydroxyl-containing organic compound with carbon monoxide in the presence of a ruthenium catalyst.
2. Description of the Art
Isocyanates such as toluene diisocyanate (TDI) and 4,4-diisocyanato diphenyl methane (MDI) are used commercially in the preparation of urethane polymers. The present commercial technology for the preparation of these isocyanates utilizes phosgene, which is costly, toxic, corrosive, and difficult to handle. It is thus understandable that a great deal of recent research has been directed toward different methods for preparing isocyanates, especially TDI and MDI.
Various patents have disclosed methods for carbonylating nitrogen-containing organic compounds, e.g. nitro compounds, amines, azo- and azoxy compounds to either isocyanates or urethanes in the presence of a platinum group metal-containing catalyst; usually a palladium or rhodium-containing catalyst, and most often a palladium halide-containing catalyst. (The urethanes can be decomposed to yield the corresponding isocyanates.) Generally, a cocatalyst (promoter) or a coreactant has been utilized in combination with the aforementioned platinum group metal-containing catalysts; Lewis acids, Lewis bases, oxidizing agents, reducing agents, etc. have been used as cocatalysts or coreactants in the platinum group metal-catalyzed carbonylation of nitrogen-containing organic compounds. It is important to note that the vast majority of the research on the carbonylation of nitrogen-containing organic compounds has been directed to catalysis by rhodium or palladium-containing catalysts; especially palladium halide-containing catalysts.
In references which teach the use of palladium catalysts, for the above reaction, generally the alcohol of choice is ethanol. See, for example, U.S. Pat. Nos. 3,531,512; 3,993,685; 4,134,880; 4,186,269; 4,219,661; 4,262,130; 4,297,501 4,304,922 and 4,339,592. In U.S. Pat. No. 4,297,501, ethanol is demonstrated to provide improved selectivity to the urethane as compared to methanol. In U. S. Pat. No. 4,134,880, methanol, ethanol and various higher alcohols are demonstrated to give similar selectivities to the urethane.
European Patent Application No. 86,281 teaches the use of methanol and a catalyst comprising palladium and a specific ligand having two moieties selected from the group consisting of nitrogen, phosphorus, arsenic and antimony. The novelty of this invention is predicated on the use of the specific ligand disclosed in the patent application in combination with palladium as a catalyst.
In the references which teach the use of rhodium catalysts for the above reaction, the use of methanol and other alcohols in the conversion of the above nitrogen-containing compounds to urethanes, in the presence of carbon monoxide, is disclosed. Again, generally the alcohol of choice is ethanol. See for example, U.S. Pat. Nos. 3,993,685; 4,134,880; 4,186,269; and 4,304,922. In U.S. Pat. No. 3,338,956, methanol is stated as a preferred alcohol, but no reason for preference is given in the disclosed, rhodium halide catalyzed process. (There is no demonstration, by example, of ethanol, therefore there is no basis for the preference of methanol, especially in view of the patents that teach ethanol is preferred). Similarly, methanol is allegedly preferred in the urethane process disclosed in U.S. Pat. No. 3,448,140, but variations in selectivity appears to be dependent on the catalyst metal rather than the alcohol. (This reference discloses Group VIII metals demonstrate varying efficiencies for catalyzing the conversion of a nitrogen-containing compound to urethane in the presence of an alcohol and carbon monoxide.) See also U.S. Pat. Nos. 3,454,620 and 3,467,694, wherein methanol is an alcohol demonstrated as useful in the above process but the effect of methanol in comparison to other alcohols on the yield is not disclosed.
In companion patent applications, U.S. Pat. Nos. 4,052,420 and 4,052,437, the lower alcohols and phenols are indicated to be preferred for reacting nitrogen-containing compound and hydroxyl-containing compounds with carbon monoxide to obtain urethanes. The process disclosed in these references is catalyzed by a specific form of rhodium, i.e. amorphous rhodium oxide, and preferably requires either a two stage conversion (wherein the second stage is at a higher temperature and pressure than the first stage) or a nitrile-containing solvent. Methanol appears to increase selectivity to urethane at the higher temperature utilized in the second stage, but ethanol is preferred in the first stage. However, due to the unpredictability of catalysis this increase must be restricted to the specific catalyst and solvent utilized in the process disclosed. (Note that the patentee, at column 9, lines 8-15, of U.S. Pat. No. 4,052,420, indicates that it is not understood why ethanol is preferred in the first stage and methanol in the second stage of this process.)
Finally, British Pat. No. 1,089,132 teaches a rhodium catalyzed process for converting a nitrogen-containing organic compound and an alcohol to urethane in the presence of carbon monoxide, wherein methanol is said to be the preferred alcohol. However, the data in this patent indicates that n-butanol demonstrates increased selectivity to urethane as compared to methanol. (There is no data on ethanol provided, therefore one can not determine why methanol would be preferred over ethanol in the disclosed process.)
In the few references which suggest that ruthenium compounds are suitable catalysts for the carbonylation of nitrogen-containing organic compounds to the corresponding urethanes or isocyanates, the catalyst is either a ruthenium halide, or a halide-containing moiety is combined with the ruthenium compound to provide the active catalyst. For example, in U.S. Pat. Nos. 3,660,458; 4,134,880; 4,186,269; and 4,304,922 the ruthenium compound that has demonstrated catalytic activity is ruthenium chloride. (As noted above, in the latter three patents wherein urethanes are prepared, ethanol is disclosed as the preferred alcohol.) In U.S. Pat. Nos. 3,461,149 and 3,979,427 ruthenium-on-alumina is treated with halide-containing compounds, such as ferric chloride or 1,1,2-trichloro-1,2,2,-trifluoroethane, to provide a heterogeneous catalyst.
Another example of a heterogeneous ruthenium catalyst for the preparation of aromatic isocyanates may be found in U.S. Pat. No. 3,737,445. This patent discloses a gas-phase process for reacting carbon monoxide with an aromatic nitro or nitroso compound to yield an aromatic isocyanate.
Ruthenium compounds have been utilized in the reduction of organic nitro compounds to the corresponding amines with mixtures of hydrogen and carbon monoxide. It was reported in U.S. Pat. No. 3,729,512 that the reduction of the organic nitro compound with carbon monoxide and ethanol, in the absence of H.sub.2, resulted in a mixture of amine and a urethane. The patentee was not concerned with the preparation of a urethane product; therefore, there was no attempt to increase the selectivity above the approximately 22 percent, urethane, that was obtained.
In the ruthenium-catalyzed processes described in above references, when an alcohol was included in the reaction mixtures, to yield a urethane reaction product, it was either ethanol (U.S. Pat. Nos. 4,186,269; 3,304,992; and 3,729,512) or isobutanol (U.S. Pat. No. 4,134,880).